The proposed study will investigate changes in the hearing capabilities and ear in aging zebrafish (Danio rerio). This teleost species has become a useful and important vertebrate model system for genetic studies, and it is of special interest since many features of the zebrafish ear, including the structure and function of the sensory receptors, are homologous to those in mammals. Among the numerous known zebrafish mutants, over 50 affect the ear as their primary target, while others affect the ear secondarily. Many of these mutations resemble those found in other vertebrates. Zebrafish have the added advantages, compared to the most popular genetic model for hearing studies, mice. These include their: having large broods; reaching sexual maturity at a relatively young age; having clear embryos; and being low cost to obtain and maintain in the laboratory. This study will investigate specific changes in the aging nervous system of zebrafish, starting before the fish reach sexual maturity and continuing through sexual maturity and into old age. The results will provide baseline data for understanding changes in the auditory system of the aging zebrafish, and potentially provide a model system in which we can apply known genetic changes in the ear to better understand the ways in which the vertebrate ear develops and ages. The study will test whether auditory sensitivity of zebrafish remains stable from a young age until a period past sexual maturity, but then get poorer in older animals. It will also determine the pattern of sensory hair cell and eighth nerve proliferation in post-embryonic fish. Auditory sensitivity will be determined using a classical conditioning paradigm that has been successfully used for other species. Hair cell addition will be done using flourescent markers and confocal microscopy.